An image sensor is a semiconductor device for converting an optical image into an electrical signal. The image sensor is roughly classified as a charge coupled device (CCD) image sensor or a complementary metal oxide semiconductor (CMOS) image sensor (CIS).
In an image sensor, a photodiode is typically formed in a substrate with readout circuitry using ion implantation. As the size of a photodiode reduces more and more for the purpose of increasing the number of pixels without an increase in a chip size, the area of a light receiving portion reduces, so that an image quality reduces.
Also, since a stack height does not reduce as much as the reduction in the area of the light receiving portion, the number of photons incident to the light receiving portion also reduces due to diffraction of light, called airy disk.
As an alternative to overcome this limitation, an attempt of forming a photodiode using amorphous silicon (Si), or forming a readout circuitry in a Si substrate and forming a photodiode on the readout circuitry using a method such as wafer-to-wafer bonding has been made (referred to as a “three-dimensional (3D) image sensor). The photodiode is connected with the readout circuitry through a metal line.
However, for a 3D image sensor, when the photodiode is formed on the substrate having the readout circuitry, a height difference is generated between the top surface of the photodiode and the top surface of the substrate. Particularly, since the lateral sides of some of the photodiodes formed in the edge region of the chip are exposed, undesired light is incident to the lateral sides and so light sensitivity may reduce.
Meanwhile, according to a related art, when the surface voltage of the photodiode is lowered by incident light, the surface voltage of a voltage sensing portion is also lowered simultaneously. After that, when a transfer transistor Tx is opened and then closed, voltages of the source and the drain of the transfer transistor become equal to each other, and a potential difference of the drain is amplified through a drive transistor. According to the related art, since the both the source and the drain of the transfer transistor are heavily doped with N-type impurities, a charge sharing phenomenon occurs. When the charge sharing phenomenon occurs, the sensitivity of an output image is reduced and an image error may be generated.
Also, according to the related art, because a photo charge does not swiftly move between the photodiode and the readout circuitry, a dark current is generated or saturation and sensitivity are reduced.